The present invention relates to hydraulically assisted steering systems and more particularly to such a system in which a hydraulic booster pump can be, as a function of vehicle speed, selectively driven either from the vehicle's primary motive power source or from an electrical motor, thereby to reduce energy consumption at higher speeds when steering effort is typically low.
Hydraulic power steering systems of the direct engine driven type are very energy inefficient, particularly if the hydraulic servo system is of the open center valving type which allows a substantial hydraulic fluid flow even when the steering wheel is not being moved, i.e. when the vehicle is traveling straight and level. While energy efficiency can be improved by utilizing so-called closed center servo valving, it has not been possible to reliably manufacture such systems with a steering "feel" which is acceptable to consumers.
While it has been recognized that overall energy efficiency could be improved by driving a power steering hydraulic pump from an electric motor, such is system is not commercially feasible because of the very large motor which would be required to provide the power required under low speed operation e.g., during parking. Systems have also been heretofore proposed for initiating electrical motor energization of a power steering pump in the event that the vehicle's engine fails or stalls. Such systems are, for example, disclosed in U.S. Pat. Nos. 3,280,557; 3,434,282; 3,528,522; 3,734,226; 3,901,342; 3,954,147; and 5,299,650.
Among the several objects of the present invention are the provision of an assisted steering system which is energy efficient; the provision of such a system which can provide the power required for assisted steering during parking; the provision of such a system which reduces energy consumption during highway travel; the provision of such a system which is highly reliable and which is of relatively simple and inexpensive construction. Other objects and features will be in part apparent and in part pointed out hereinafter.